Method for etching multi-layer epitaxial material

ABSTRACT

A single-step wet etch process is provided to isolate multijunction solar cells on semiconductor substrates, wherein the wet etch chemistry removes semiconductor materials nonselectively without a major difference in etch rate between different heteroepitaxial layers. The solar cells thus formed comprise multiple heterogeneous semiconductor layers epitaxially grown on the semiconductor substrate.

This application claims benefit under 35 U.S.C. §119(e) to U.S.Provisional Application No. 61/561,179 filed on Nov. 17, 2011, which isincorporated by reference in its entirety.

This application relates to a single-step wet etch process and the useof the process to isolate multijunction solar cells on semiconductorsubstrates, wherein the wet etch chemistry removes semiconductormaterials nonselectively without a major difference in etch rate betweendifferent heteroepitaxial layers. The solar cells thus formed comprisemultiple heterogeneous semiconductor layers epitaxially grown on thesemiconductor substrate.

BACKGROUND

The present invention relates to the field of multijunction solar cellsincorporating a plurality of heterogeneous layers of epitaxial materialon semiconductor substrates. More particularly, the present inventionrelates to an improved method of patterning epitaxial layers using asingle step wet etch process.

Solar cells are important renewable energy sources that have becomewidely deployed for both space and terrestrial applications. Today, thehighest efficiency solar cells are realized using the multijunctionsolar cell approach. Multijunction solar cells typically consist of two,three, or more junctions, i.e., subcells, that are serially connected ina stack, as illustrated in FIG. 1A. The junctions are typically realizedby growing a plurality of heteroepitaxial layers 9 on a semiconductorsubstrate 12. Each junction 114, 115, and 116 is designed to absorb froma separate portion of the solar energy spectrum, allowing for solarenergy conversion with high efficiency. The junctions are separated byinterconnection regions 117 and 118. The epitaxial layers 9 that make upthe multijunction solar cells are chosen from a variety of semiconductormaterials with different optical and electrical properties in order toabsorb different portions of the solar energy spectrum.

Fabrication of multijunction solar cells is conventionally carried outon the wafer scale using conventional semiconductor processing methodsthat are known to those skilled in the art. A summary of processingsteps for making a typical multi junction solar cell is found inDanzilio et al., cited below.

An important step in the fabrication of multijunction solar cells,illustrated in FIG. 1B, is the isolation of individual cells 100 on asemiconductor substrate 12, herein referred to as “mesa isolation.” Mesaisolation is done to eliminate electrical contact between adjacent solarcells on the same wafer, which then allows the mechanical separation ofthe individual cells 100 (also referred to as singulation) to take placewithout disturbing the edges of each individual cell.

In the mesa isolation step, the epitaxial layers 9 must be removed onall four edges of the individual solar cell chips, as can be seen inFIG. 1A and FIG. 2 . If the bottom-most junction in the stack uses thesubstrate 12 as seen in FIG. 2 as the base region of the junction (e.g.,germanium), then mesa isolation must extend into the substrate region,resulting in partially etched substrate 12, such that the etch depthinto the substrate exceeds the minority carrier diffusion length in thatbase region.

Mesa isolation of cells can be achieved by using a number of techniquesincluding, but not limited to, dry etching, partial (or full) cut usingtechniques such as saw or laser dicing, and wet etching. Thesetechniques, as used in the prior art, can be reviewed briefly asfollows:

Dry Etching: Dry etching is the removal of semiconductor material byexposing the material to plasma of reactive gases in vacuum chambers.Dry etching is a well-established processing technique in thesemiconductor industry. However, it has found limited use in solar cellmanufacturing due to low throughput and high costs involved in equipmentsetup and maintenance. Consequently, dry etching is not typically usedfor mesa isolation of multijunction solar cells.

Partial Cut: The partial cut technique uses a dicing saw or laser beamto cut partially through the wafer to isolate solar cells electrically.Mechanical removal of semiconductor material results in chipping anddamage on sidewalls, which leads to poor electrical performance of thechip. It is difficult to control the actively absorbing area of thesolar cell, resulting in a variation in performance. Furthermore,partial cut is also a low throughput technique. Nevertheless the partialcut technique is relatively cost-effective and is sometimes used.

Wet Etching: Wet etching is the removal of semiconductor material usingchemicals in liquid phase. Wet etching is the preferred method found inthe prior art, because it is cost effective, does not requiresophisticated processing equipment, and wafer throughput is high. Thepresent invention is a wet etching technique for multijunction solarcell processing.

Multijunction solar cells are formed of multiple epitaxial layers withdifferent electrical, optical, and chemical properties. Semiconductormaterials used in multijunction solar cells include, but are not limitedto, indium gallium phosphide, indium phosphide, gallium arsenide,aluminum gallium arsenide, indium gallium arsenide, dilute nitridecompounds, and germanium. For ternary and quaternary compoundsemiconductors, a wide range of alloy ratios can be used.

There are several wet etch chemistries commonly used in compoundsemiconductor manufacturing. A comprehensive list of wet etchants, alongwith etch rates and selectivity relationships was published by Clawson,Materials Science and Engineering, 31 (2001) 1-438. Typically, the wetetchants used for etching one class of semiconductor material areselective and will not etch certain other classes of semiconductormaterial. The selectivity of a wet etchant may also depend on alloyconcentration of the compounds. Consequently, etching a full stack ofepitaxial layers in multijunction solar cells for mesa isolationtypically has required application of multiple wet etch chemistries.

Using multiple applications of selective (or nonselective) etchantstypically results in jagged, non-smooth, and/or irregular mesa sidewallprofile. FIG. 4 depicts an example of a multijunction solar cell foundin the prior art, as would be produced by an image of a scanningelectron microscope. In the fabrication of the solar cell shown in FIG.4, different etch chemistries were used to etch each of the junctions114, 115, and 116. Due to differences in chemistries and selectivityrelationships, each of the three junctions and the substrate 112 has adistinctively different etch profile, resulting in the jagged,non-smooth, and/or irregular shape shown in FIG. 4. The shape of mesasidewalls will vary depending on the semiconductor materials used in thesolar cell and the chemicals used to etch them. Typically, such jaggedmesa sidewall profiles result in larger sidewall surface areas (for agiven mesa size and etch depth) compared to a uniform sidewall profile.Such larger sidewall surface areas may result in a higher leakagecurrent on the perimeter of the solar cell, which in turn may result inreduced open circuit voltage and efficiency in multijunction solarcells.

In addition to perimeter leakage current, the use of multiple etchantshas other disadvantages compared to single-etch chemistries including,for example:

1. Longer processing time.

2. Increased difficulty in controlling the etch rate and relativeundercut of layers.

3. More chemical, water, and energy consumption during fabrication.

4. More chemical waste generation.

5. Multiple photolithography steps may be required to prevent damage tothe sidewalls.

6. Uneven etching of different semiconductor layers.

Conventional processes have not been adequate to fabricate acost-effective, high performance multijunction solar cell. Consequentlythere have been attempts to find nonselective etchants. The priorattempts are briefly described.

Turala et al. (CPV-7 Las Vegas, Nev. 2011) disclose a method using ahigh-viscosity bromine solution to etch III-V materials used inmultijunction solar cells on germanium substrates. The solution proposedby Turala et al., however, requires the use of a silicon nitride hardmask, which must be deposited using plasma enhanced chemical vapordeposition. Using dielectric hard mask is not the preferred method infabrication of solar cells because the use of photoresist masking,common in all semiconductor manufacturing, is much more cost-effective.

In a different approach, Zaknoune et al., J. Vac. Sci. Technol. B 16,223 (1998), report an etching procedure that is nonselective for galliumarsenide and aluminum gallium indium phosphide, wherein the aluminumgallium indium phosphide quaternary compound has 35% aluminum phosphide,15% gallium phosphide, and 50% indium phosphide. The etching proceduredescribed by Zaknoune et al. uses a diluted solution of hydrochloricacid, iodic acid, and water to etch 300 nm of the quaternary compoundgrown on gallium arsenide substrate using a photoresist mask. The mainapplication areas described in the paper by Zaknoune et al. areheterojunction bipolar transistors (HBT), various quantum well lasers(QWL), and high electron mobility transistors (HEMT) for which largeconduction and valance band discontinuities are required. These devicesare majority carrier devices wherein the large bandgap materials aretypically used as barrier materials for majority carriers. Zaknoune etal. describes a system with one layer of epitaxy and does not recognizeany sidewall problem related to multilayer epitaxy that ischaracteristic of solar cells.

The device requirements for multijunction solar cells are significantlydifferent than for HBTs, QWLs, and HEMTs, largely because multijunctionsolar cells are minority carrier devices. Consequently the proceduredescribed by Zahnoune et al. has no direct application to etchingmultijunction solar cell structures, which include a wide variety ofsemiconductor materials with a wide range of bandgaps (typically 0.67 eVto 2.25 eV).

The requirements for the mesa isolation step in the fabrication ofmultijunction solar cells can be summarized as follows:

1. Etchants that do not affect standard photoresist materials, such thatconventional photolithography techniques can be used to define mesapatterns on semiconductor substrates and the exposed areas can beremoved using wet etch chemistry. In particular, dielectric hard maskscan thereby be avoided when defining mesa patterns.

2. A nonselective etch that etches all materials in the range ofbandgaps from 0.67 eV (germanium) to 2.25 eV (gallium phosphide). FIG. 3shows some of these semiconductor materials and their alloys in therange of bandgap energies.

3. Small sidewall surface area to result in reduced leakage current.

4. Ability to change and control the relative etch rates of epitaxiallayers within the solar cell, for example through changing thecomposition of the etchant. An etchant that etches all materials in thebandgap range with similar etch rate may be preferred.

5. Using a single mixture of chemicals such that the wet etch for mesaisolation can be completed in a single step with minimal wastegeneration and water and energy consumption.

What is needed is a fabrication process better adapted to multijunctionsolar cell manufacturing.

SUMMARY

According to the invention, a single-step wet etch process is providedto isolate multijunction solar cells on semiconductor substrates priorto singulation, wherein the wet etch chemistry removes semiconductormaterials nonselectively without a major difference in etch rate betweendifferent heteroepitaxial layers. The solar cells thus formed comprisemultiple heterogeneous semiconductor layers epitaxially grown on thesemiconductor substrate without ragged sidewalls.

The present invention includes a mixture of the following chemicals torealize mesa isolation: (1) iodic acid (HIO₃); (2) hydrochloric acid(HCl); and (3) water (H₂O)

The mole ratios of these chemicals and the temperature of the chemicalmixture can be varied to change the overall etch rate and the relativeetch rates of different epitaxial materials. Agitation during etchingmay be used to control the etch rate and uniformity.

In application, examples of processes taking place at the mesa isolationstep are listed as follows:

1. Patterning photoresist 10 using standard photolithography techniquesto define mesa areas 6 on the substrate (FIG. 5A).

2. Etching the wafers in a mixture of the above listed chemicals toremove the epitaxial material 11 and portion of the substrate 12 in theexposed areas between adjacent mesa structures (FIG. 5B).

3. Removing the photoresist layer 10 (FIG. 5C).

The process steps of the present invention constitute the mesa isolationstep in fabrication of multijunction solar cells. The steps describedherein are not to be taken in a limiting sense. Other embodiments may beused and structural or logical changes in the process may be madeprovided that removal of the epitaxial material in exposed areas isachieved by using a mixture including iodic acid, hydrochloric acid, andwater. It is to be understood that the present invention is a processmodule that can be inserted in all multijunction solar cell processesthat require mesa isolation.

The present invention solves a number of critical problems that are notaddressed in the prior art, as described in the background section:

1. The technique etches all semiconductor materials and alloys typicallyused in multijunction solar cells including but not limited to:

-   -   Germanium    -   Indium phosphide    -   Aluminum phosphide    -   Gallium phosphide    -   Indium arsenide    -   Aluminum arsenide    -   Gallium arsenide    -   Dilute nitride compounds

2. The etch rate does not depend on crystallographic orientation.

3. The method does not significantly etch common photoresist materialstypically used in standard photolithography.

4. The technique does not significantly etch dielectrics, such as commondielectric layer stacks used as anti-reflection coatings.

5. The technique employs a single mixture of chemicals and the mesaisolation step can be realized in a single step.

6. The mesa sidewall areas obtained through the use of the presentinvention are generally smaller compared with those found in the priorart.

The invention will be better understood by reference to the followingdetailed description in connection with the accompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional diagram of a multijunction solar cell inwhich the invention could be used.

FIG. 1B is a top plan view of cells as found in the prior art.

FIG. 2 is a close-up view of a portion of the mesa isolation processedstructure of the prior art. Individual cells on a semiconductorsubstrate are isolated by removing grown epitaxial layers and a portionof the substrate in areas between adjacent mesa structures.

FIG. 3 is a graph showing the lattice constant-bandgap relationship ofcommon compound and elemental semiconductors and the alloys typicallyused in multijunction solar cells.

FIG. 4 is a cross-sectional view of an exemplary multijunction solarcell of the prior art.

FIGS. 5A, 5B, and 5C show results of processing steps in accordance withthe present invention.

FIG. 6 is a cross-sectional view of a multijunction solar cellmanufactured using the wet etch process of the present invention.

FIGS. 7A, 7B, and 7C show results of processing steps of one embodimentof the present invention wherein a patterned photoresist mask 21 is usedto pattern the mesa structures.

FIGS. 8A, 8B, and 8C show results of processing steps of anotherembodiment of the present invention wherein patterned photoresist 10 isfirst used to pattern dielectric 21, and thereafter, retained duringmesa etch.

DETAILED DESCRIPTION

FIG. 1B and FIG. 2 illustrate of a portion of a mesa isolation processedsolar cell structure 100 on a wafer 12. In top view, the structures ofthe prior art and of the present invention are indistinguishable.Individual solar cells 100 on the semiconductor substrate 12 areisolated by removing grown epitaxial layers and a portion of thesubstrate 12 in areas 8 (etched regions) between adjacent mesastructures. Bus bars 22 connect grid lines 2. The mesa structure 6 isbounded by edges along the boundaries of the etched regions 8 ashereinafter explained.

FIG. 3 is a graph showing the lattice constant-bandgap relationship ofcommon compound and elemental semiconductors and the alloys typicallyused in multijunction solar cells. In mesa isolation it is a requirementto etch through all of the semiconductor materials used in multijunctionsolar cells. Whereas the prior art as represented by FIG. 4 yields aninconsistent cross section during etch, the present invention, as shownin FIG. 6, does not.

The mixture according to certain embodiments of the wet etch process ofthe invention, comprises iodic acid, hydrochloric acid, and waterprepared in the molar ratios of 1:62:760, respectively. The said molarratios of iodic acid and hydrochloric acid can be within a variance of±5.0%, such that the molar ratios in the mixture are within the ranges(0.95-1.05):(59-65):760, for iodic acid, hydrochloric acid, and water,respectively. Preparation in the laboratory or manufacturing process isin a 1:2:3 ratio by volume, wherein the aqueous solution of hydrochloricacid is 38.0%±3.0% by weight and the aqueous solution of iodic acid is6.6%±1.0% by weight. It is within the contemplation of the invention touse another solute or liquid mixtures besides water in the wet etchprocess, although water is the most readily available. Similarly, otheracids of different molar concentration could be substituted forhydrochloric acid to yield the same result. However, it is the iodicacid HIO₃ in the above concentration range that is consideredefficacious for the purposes of this invention to produce substantiallyinwardly curved sidewalls through all heterogeneous layers of epitaxy.

FIGS. 5A, 5B, and 5C show processing steps in accordance with thepresent invention as previously summarized. The layers of amultijunction solar cell are represented by heteroepitaxial materialstack 11 on an unmodified substrate 12. Photoresist 10 overlays theepitaxial stack 11 (FIG. 5A). When etched according to the invention(FIG. 5B) in a reservoir of the aqueous etch solution, heteroepitaxialstack 11 is transformed into individual elements 13 that form cells 100(FIG. 1) penetrating the substrate, now designated 121, and producingmesa structures 6, as hereinafter depicted in FIG. 6 according to theinvention. (Not shown is the optional process of protecting theunderside of the substrate 12 against the etch solution.)

The resulting cross-sectional shape (FIG. 6) after the mesa isolationstep is a side wall profile characterized by a substantially inwardlycurved profile, that is, having a substantially macroscopically smoothsurface without significant undercutting of a junction region comparedto other junction regions. As compared with the prior art, asubstantially inwardly curved profile is indicative of nonselective etchproduced by the wet etch process of the invention. Interconnectionregions 17 and 18 between junctions 14, 15, and 16 often exhibitdifferent etch rates compared to the semiconductors used in thejunctions. As shown in FIG. 6, methods provided by the presentdisclosure produce a mesa side wall profile that is characterized by amacroscopically smooth profile from junction 14 and gradually wideningtoward and along substrate 121. The mesa sidewall profile is furthercharacterized by minimal undercutting beneath the uppermostinterconnection layer and interconnection layer 17, and a modestetch-back at interconnection layer 18. In particular, aluminumcontaining layers may show a larger undercut compared to other layers.It is to be understood that traces of iodine may be found on etchedsurfaces 19 (FIG. 6) as a result of the present invention, includingsidewalls of mesa structures 9 and exposed portions of the substrate 121(FIG. 5C).

In certain embodiments the wafers are agitated in the etch solution tocontrol etch rate and provide etch uniformity across wafers.

In another embodiment, anti-reflection coating (ARC) is used as adielectric hard mask for mesa isolation. The process steps in thisembodiment are depicted in FIG. 7. Anti-reflection coating deposition 20is done before the mesa isolation step and patterned ARC 21 is used toprotect the solar cells during the removal of heteroepitaxial layersaround the cells using the present invention. In a variation of thisembodiment (FIG. 8), the ARC 20 is patterned using photoresist 10. Afterthe ARC etch is completed the photoresist 10 is retained during mesaisolation as well.

In certain embodiments of the invention, the volumetric ratio ofhydrochloric acid in the mixture is 10%-50% and the volumetric ratio ofiodic acid in the mixture is 10%-50%, wherein the aqueous solution ofhydrochloric acid is 38.0%±3.0% by weight and the aqueous solution ofiodic acid is 6.6%±1.0% by weight. It is to be understood the same molarratios of the constituent chemicals can be provided using differentvolumetric ratios with different molarities in the aqueous solutionsused. During processing, the temperature of the mixture is maintainedbetween 10° C. and 140° C.

In another specific embodiment of the invention, the volumetric ratio ofhydrochloric acid is 30%-35% and the volumetric ratio of iodic acid is14%-19%, using the said molarities in the aqueous solutions of theconstituent chemicals, and the temperature of the mixture is maintainedbetween 30° C. and 45° C.

The etching methods provided by the present disclosure can be used tofabricate solar cells including multijunction solar cells. Accordingly,solar cells including multijunction solar cells fabricated using themethods disclosed herein are provided.

In certain embodiments, a solar cell device comprises a multijunctionphotovoltaic cell comprising a first subcell and a second subcelloverlying the first subcell, wherein the multijunction photovoltaic cellis characterized by mesa sidewalls formed by a nonselective wet etchprocess, wherein the mesa sidewalls are substantially inwardly curved.

In certain embodiments of a solar cell, at least one of the firstsubcell and the second subcell comprises a base layer comprising analloy comprising one or more elements from group III of the periodictable, nitrogen, arsenic, and an element selected from Sb, Bi, and acombination thereof; and the first subcell and the second subcell aresubstantially lattice matched.

In certain embodiments of a solar cell, the first subcell and the secondsubcell are substantially lattice matched to a material selected fromSi, Ge, SiGe, GaAs, and InP.

In certain embodiments of a solar cell, the first subcell comprises abase layer selected from GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb,GaNAsBi, and GaNAsSbBi.

In certain embodiments of a solar cell, the solar cell comprises agallium arsenide-containing material underlying the first subcell.

In certain embodiments of a solar cell, the solar cell comprises agermanium-containing material underlying the first subcell.

In certain embodiments of a solar cell, the first subcell comprises agermanium-containing material.

In certain embodiments of a solar cell, the nonselective wet etchprocess used to form the mesa sidewalls comprises the use of a singleetchant mixture comprising hydrochloric acid, iodic acid, and water.

In certain embodiments of a solar cell, the etchant mixture used to formthe mesa sidewalls comprises a volumetric ratio of hydrochloric acid of10% to 50%; and a volumetric ratio of iodic acid of 10% to 50%; and theetchant mixture is characterized by a temperature from 10° C. to 140° C.

In certain embodiments of a solar cell, the etchant mixture used to formthe mesa sidewalls comprises a volumetric ratio of hydrochloric acid of30% to 35%; and a volumetric ratio of iodic acid of 14% to 19%; and theetchant mixture is characterized by a temperature from 30° C. to 45° C.

The invention has been explained with respect to specific embodiments.Other embodiments will be evident to those of ordinary skill in the art.Therefore, the invention is not intended to be limited, except asindicated by the appended claims.

What is claimed is:
 1. A method comprising the steps of: providing awafer comprising a substrate and a stack of grown heteroepitaxial layerswherein the stack comprises one or more semiconductor junctions of solarcells; patterning the wafer with a mesa etch pattern usingphotolithography; and etching in exposed areas the stack and part or allof the substrate according to the mesa etch pattern using a singleetchant mixture comprising hydrochloric acid, iodic acid, and water, foryielding mesa isolation of individual solar cells characterized by asubstantially inwardly curved sidewall profile.
 2. The method of claim1, wherein the etchant mixture comprises: A volumetric ratio ofhydrochloric acid of 10% to 50%; and a volumetric ratio of iodic acid of10% to 50%; wherein the mixture has a temperature 10° C. to 140° C. 3.The method of claim 1, wherein the etchant mixture comprises: avolumetric ratio of hydrochloric acid of 30% to 35%; and a volumetricratio of iodic acid of 14% to 19%; wherein the mixture has a temperatureof 30° C. to 45° C.
 4. The method of claim 1, wherein patterningcomprises using a photoresist, using a dielectric hard mask, or usingboth a photoresist and a dielectric hard mask.
 5. The method of claim 1,wherein patterning comprises using a photoresist to pattern a dielectrichard mask, and the photoresist is retained during removal of theheteroepitaxial layers during mesa etching.
 6. The method of claim 1,wherein etching comprises agitating the wafer.
 7. A solar cell devicecomprising: a multijunction photovoltaic cell comprising a first subcelland a second subcell overlying the first subcell, wherein: themultijunction photovoltaic cell is characterized by mesa sidewallsformed by a nonselective wet etch process, wherein the mesa sidewallsare substantially inwardly curved.
 8. The solar cell device of claim 7,wherein at least one of the first subcell and the second subcellcomprises a base layer comprising an alloy comprising one or moreelements from group III of the periodic table, nitrogen, arsenic, and anelement selected from Sb, Bi, and a combination thereof; and the firstsubcell and the second subcell are substantially lattice matched.
 9. Thesolar cell device of claim 7, wherein the first subcell and the secondsubcell are substantially lattice matched to a material selected fromSi, Ge, SiGe, GaAs, and InP.
 10. The solar cell device of claim 7,wherein the first subcell comprises a base layer selected fromGaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi, and GaNAsSbBi. 11.The solar cell device of claim 7, comprising a galliumarsenide-containing material underlying the first subcell.
 12. The solarcell device of claim 7, comprising a germanium-containing materialunderlying the first subcell.
 13. The solar cell device of claim 7,wherein the first subcell comprises a germanium-containing material. 14.The solar cell device of claim 7, wherein the nonselective wet etchprocess comprises the use of a single etchant mixture comprisinghydrochloric acid, iodic acid, and water.
 15. The solar cell device ofclaim 14, wherein, the etchant mixture comprises: a volumetric ratio ofhydrochloric acid of 10% to 50%; and a volumetric ratio of iodic acid of10% to 50%; and the etchant mixture is characterized by a temperaturefrom 10° C. to 140° C.
 16. The solar cell device of claim 14, whereinthe etchant mixture comprises: a volumetric ratio of hydrochloric acidof 30% to 35%; and a volumetric ratio of iodic acid of 14% to 19%; andthe etchant mixture is characterized by a temperature from 30° C. to 45°C.